Page 63 - RSE - Results of the Apollon Project

Results of the APolloN PRoject ANd coNceNtRAtiNg PhotovoltAic PeRsPective

The indoor tests aim to determine the voltage temperature coeffcient as a function of the dark current. Once
such relationship is known, the operating junction temperature in outdoor conditions can be determined simply by
measuring the electrical variables Voc and Isc, that are associated to any outdoor I-V curve.
The APM is based upon a single junction equivalent modelling of the Multi-Junction CPV module whose parameters
(equivalent ideality factor neq, series resistance Rs, inverse saturation current Ioeq) have to be determined by an
appropriate identifcation procedure.
The APM requires that, during the indoor tests, the CPV module is equipped with a temperature sensor placed
as close as possible to the solar cell to allow measuring the cell temperature when the thermal heating is applied
either by injecting a current or by using a climatic room.
The APM method is characterized by the following steps:

1. measurement of the dark I-V curve at a given temperature Tj, by using a dedicated thermocouple;
2. identifcation of the electrical parameters of the single junction equivalent model;
3. calculation of the indoor I-V curve depurated of series resistance voltage drop;
4. determination of the voltage-temperature coeffcient ß as a function of the dark current values;
5. estimation of junction temperature in experimental conditions.

MEASUREMENT OF THE DARK I-V CURVE AT A GIVEN TEMPERATURE TJ, By USING A DEDICATED
THERMOCOUPLE
Before detecting the dark I–V curve, the module must be kept in steady-state conditions with indoor temperature
equal to 25°C. The dark I-V curve is taken by a I–V tracer, which generates a ramp of current pulses varying from 0 to
a maximum value Imax; such value should be equal to the nominal short circuit current value that is produced in
the solar cell under the reference spectrum ASTM G173D. In order to keep the cell temperature constant, preventing
the injected current from heating the cell junction, each current pulse width must be lower than 10 ms.

IDENTIFICATION OF THE ELECTRICAL PARAMETER OF THE SINGLE JUNCTION EqUIVALENT MODEL
The module dark I-V curve equation is:

KT (1)


V = N ––––– ∙ neq ∙ In(I) – neq ∙ In (Ioeq) + Rseq ∙ I
q

where:
p K is the Boltzmann constant;
p q is the electron charge;
p T is the temperature in Kelvin degrees;
p n is the equivalent cell ideality factor;
eq
p Io is the equivalent inverse saturation current;
eq
p Rs is the equivalent cells series resistance;
eq
p N is the number of series connected solar cells.
FiguRE 69. Results of module parameter identifcation

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